A mixed pseudo-orthogonal frequency coding (Mixed-POFC) structure is
proposed as a new spreadspectrum technique in this paper, which employs
frequency and time diversity to enhance tag properties and balances the
spectrum utilization and code diversity. The coding method of SAW RFID
tags in this paper uses Mixed-POFC with multi-track chip arrangements.
The cross-correlation and auto correlation of Mixed-POFC and POFC are
calculated to demonstrate the reduced overlap between the adjacent
center frequencies with the Mixed-POFC method. The center frequency of
the IDT and Bragg reflectors is calculated by a coupling of modes (COM)
module. The combination of the calculation results of the Bragg
reflectors shows that compared with a 7-chip POFC, the coding number of
a 7-chip Mixed-POFC is increased from 120 to 144 with the same
fractional bandwidth of 12%. To demonstrate the validity of Mixed-POFC,
finite element analysis (FEA) technology is used to analyze the
frequency characteristics of Mixed-POFC chips. The maximum error between
designed frequencies and simulation frequencies is only 1.7%, which
verifies that the Mixed-POFC method is feasible.